1. Field of the Invention
The present invention relates to a switch for controlling signal delivery in a high frequency band wireless communication and a radio frequency (RF) system and, more specifically, to a micro-electromechanical systems switch driven by an electrostatic force and a method of fabricating the same.
2. Discussion of Related Art
In general, in a high frequency band communication system, a field effect transistor (FET), a PIN diode, or the like is used as a switching device for controlling signal delivery. These semiconductor switches are easily integrated but have for example high insertion loss, low isolation loss, and signal distortion. Therefore, a micro electromechanical systems (MEMS) switch has been widely studied to address the above problems.
The micro electromechanical systems switch is generally composed of a micro moving element that relatively moves against a substrate, and a driving element for driving the moving element. The driving element has two electrodes located to face each other and the moving element is driven by an electrostatic force generated by a voltage applied through the electrodes of the driving element. In other words, the moving element moves against the substrate in a parallel or horizontal direction, or rotates against the substrate within a predetermined angle.
FIGS. 1A and 1B are diagrams of an example of a typical cantilever type MEMS switch. FIG. 1A is a plan view of the typical cantilever type MEMS switch and FIG. 1B is a cross-sectional view taken along the line A1-A2 of FIG. 1A.
A lower electrode 2 and a signal line 3 are formed on a substrate 1, and a cantilever arm 5 fixed to the substrate 1 by a supporting unit 4 is positioned over the lower electrode 2 and the signal line 3. An upper electrode 6 is arranged on the cantilever arm 5, and a contact portion 7 for connecting a disconnected portion of the signal line 3 is formed below an end of the cantilever arm 5. The cantilever arm 5 and the upper electrode 6 have an intermediate portion formed to be narrower than other portions so that an end of the cantilever arm 5 has certain elasticity.
When a predetermined constant voltage is applied between the upper electrode 6 and the lower electrode 2, the cantilever arm 5 is bended downward by an electrostatic force generated in a capacitor structure 8 in which the upper electrode 6 and the lower electrode 2 are laminated. Accordingly, the contact portion 7 comes in contact with the disconnected portion of the signal line 3 to perform an switching operation (See U.S. Pat. No. 5,578,976 issued on Nov. 26, 1996)
In the cantilever as shown in FIGS. 1A and 1B, the signal line (inner wiring) 3 and the contact portion (short-circuit bar) 7 connected to an input and an output, respectively, are located perpendicular to each other, and only one side of the cantilever arm (dielectric layer) 5 is supported. Therefore, when the cantilever arm 5 or the upper electrode 6 is deformed by thermal expansion during a manufacturing process or operation process, it cannot move in a vertical direction as shown in FIG. 2A, but moves in a distorted manner as shown in FIG. 2B so that the contact between the signal line 3 and the contact portion 7 becomes worse. The bad contact increases contact resistance of the signal line 4, or makes signal delivery unstable, thereby degrading reliability.
FIG. 3 shows an example of a conventional membrane type MEMS switch.
A supporting frame 24, a lower electrode 14, and an opened signal line 18 are formed on a substrate 12, and a moving plate 20 constituting an upper electrode 16 is positioned over the lower electrode 14 and the signal line 18 with a certain gap therebetween. Further, the supporting frame 24 supports a spring 22 at both sides of the signal line 18 such that the moving plate 20 has certain elasticity.
When a predetermined driving voltage is applied to the lower electrode 14, the moving plate 20 of the upper electrode moves downward due to the electrostatic force generated between the lower electrode 14 and the upper electrode 16. Accordingly, a connection frame 34 positioned in the moving plate 20 connects the disconnected portion of the signal line 18 to perform a switch operation (See U.S. Pat. No. 6,307,452 issued on Oct. 23, 2001)
In the membrane type as shown in FIG. 3, the signal line 18 and the supporting frame 24 are located with long distance therebetween. Therefore, when the surface of the upper electrode 16 made of metal is deformed by thermal expansion during a manufacturing process or operation process, the moving plate 20 may not be in complete contact with the signal line 18 to be permanently opened between them, and stiction between the upper electrode 16 and the lower electrode 14 may occur due to a narrow distance between the upper electrode 16 and the lower electrode 14. Thus, stability and reliability of the switch is degraded.
In addition, when the moving plate 20 and the spring 22 are deformed by thermal expansion, the moving plate 20 cannot move in parallel with the substrate 12. This is because the moving plate is largely thermally expanded while the supporting frame 24 is fixed to the substrate 12 having much smaller thermal expansion than that of the moving plate 20 and accordingly there is little change in the distance between the supporting frames 24. The thermal expansion causes significant stress at a connection portion between the moving plate 20 and the spring 22, where permanent deformation is made. As a result, as the moving plate 20 is deformed, it is abnormally separated from the substrate 12 or is inclined into one side so that a normal switch operation is not performed. In addition, when the moving plate 20 moves down and is close to the substrate 12, the connection frame 34 of the moving plate 20 may be in permanent contact with the signal line 18.
In addition, both electrodes, which are applied with the voltage for generating the electrostatic force, remain very close each other in an interval of several micrometers, and it may cause a stiction problem that the moving plate 20 or the spring 22 sticks to other neighboring fixing elements, which acts as a very critical defect in the operation and reliability of the switch.
As described above, while the cantilever or membrane type MEMS switch has been proposed to address the problems of existing semiconductor switches, such as high insertion loss, low isolation loss, and signal distortion, it has low reliability and a signal isolation characteristic due to structural problems such as thermal deformation and stiction. Therefore, there is a need for a new MEMS switch capable of solving the aforementioned problems.